An optical reduction system or an erecting equal-magnification optical system is used in an image reading device, such as a scanner, facsimile, or the like, and in an image forming device, such as an LED printer or the like. In particular, as compared to when using an optical reduction system, a characteristic of an erecting equal-magnification optical system is the ease with which the entire device can be made compact.
Erecting equal-magnification optical systems have been formed by inserting rodlike lenses such as Selfoc lenses (registered trademark, Nippon Sheet Glass) or rod lenses into opaque black resin in the form of an array. Since each lens has erecting equal-magnification properties, the erecting equal-magnification properties are maintained even after the lenses are arranged as an array.
By varying the refractive power from the center of the rod towards the periphery, the above-described Selfoc lenses or rod lenses are provided with light-harvesting properties. Since these lenses need to be manufactured with a special method as compared to regular lenses, manufacturing is difficult and expensive. An erecting equal-magnification optical system using a lens array plate with convex surfaces arranged in an array has therefore been proposed (see Patent Literature 1).
Furthermore, with an erecting equal-magnification optical system using Selfoc lenses, the depth of field is narrow. In an image reading device such as a scanner, by placing an object for which an image will be read on a cover glass that is maintained at a constant distance from the optical system, the distance between the object for which the image will be read and the optical system is maintained at a desired distance. By thus maintaining the distance between the object and the optical system at the desired distance, it is possible to read an image with little blurring even with a narrow depth of field.
Depending on the object to be read, however, the read surface might not be in close contact with the cover glass, but rather might be separated therefrom. In such a case, the read image is highly blurred due to the narrow depth of field. An erecting equal-magnification optical system with an expanded depth of field has therefore been proposed (see Patent Literature 2).